As noted earlier, condensation takes place when water vapour is converted into liquid form. This process may produce dew, fog or clouds. Howsoever different these forms of condensation may be, they have two things in common.
First, for condensation to occur, saturation of air is an essential pre-requisite. Saturation can be brought about either by reducing the air temperature or by adding water vapour to the air; secondly, water vapour needs some kind of a surface on which it may condense.
For dew or frost, solid objects at the ground do this job. But when condensation occurs at some distance away from the earth’s surface, the most needed surfaces for condensation of water vapour are provided by the dust particles or aerosols, as they are called, which are referred to as condensation nuclei.
In the absence of these tiny dust particles, clouds may not possibly form unless a relative humidity of nearly 400 per cent is reached. Fortunately there is an abundance of condensation nuclei such as salt particles supplied by the oceans, microscopic dust particles and smoke etc. in the lower atmosphere.
Since these tiny particles have affinity for water, they are termed hygroscopic nuclei. Because of the presence of an abundant quantity of such water-seeking dust particles, relative humidity hardly exceeds 101 percent.
Condensation is said to be a continuous process, since some of the more powerful hygroscopic nuclei begin to attract water vapour around them at relative humidity as low as 75 per cent.
As the relative humidity reaches 100 per cent mark, the condensed water particles grow larger in size very quickly. Thus, the clouds start appearing in the sky.